Generally, viscosity is a coefficient that expresses the physical property of the fluid viscosity volume, and various types of viscometers are developed and used to measure this type of viscosity.
A viscometer called the Greenspan viscometer (Refer to M. Greenspan and F. N. Wimenitz, “An Acoustic Viscometer for Gases-I,” NBS Report 2658 (1953)), which was invented in 1953 by Greenspan and Wimenitz to measure the viscosity of gas with a design that comprises of two Helmholtz Resonators that are attached and face each other, exists, however, this viscometer relatively takes up much space and the performance shows a 38% error margin that was considered not to be suitable structure.
In 1996, K. A. Gillis came up with a more precise viscometer, compared to the Greenspan viscometer, by going through an experimental error and correction process to reduce the errors within the range of ±0.5% and derive more accurate gas viscosity measurements. (Refer to R. A. Aziz, A. R. Janzen, and M. R. Moldover, Phys. Rev. Lett. 74, 1586 (1995))
However, issues for this method existed, where the valid frequency section for this method was limited only to the low frequency domain. For example, the viscometer designed by K. A. Gillis could only be applied to low frequency under 200 Hz. The reason for this phenomenon was because the Helmholtz Resonator was applied assuming that the product of the wave number of the sound wave and the characteristic length was greatly smaller than 1. Furthermore, due to the fact that no measurements were made for liquids, the measurements were restricted to gas and the size was large that required a mass amount of fluid.
Also, U.S. Pat. No. 6,141,625 discloses a viscosity module with a crystal resonant sensor, where this module relates to a mobile viscometer that can measure the viscosity of the fluid even with a small quantity of reagent. This viscometer utilizes a disk type thin crystal film for the viscosity sensor.
To obtain this type of crystal resonant frequency, the sensor operates in thickness shear mode by positioning electrodes on the top and bottom portion of the thin film and passing a signal. If a certain liquid exists on the top surface of the crystal, then power loss occurs that cause damping in the crystal resonant frequency. Eventually, the viscosity of the liquid can be measured by checking the value of damping.
However, accurate measurements are only obtainable when a viscosity module with a crystal resonant sensor is placed horizontally, liquid is equally distributed and a large amount of liquid. In other words, the viscosity module with a crystal resonant sensor needs several ml of liquid with assuming that the volume of a single water drop is 0.04 ml and an issue exists where it is impossible to take measurements with the volume of a single water drop. Furthermore, this type of viscometer has demerits that it is impossible to make measurements of gas, since it utilizes the gravity applied on liquids.
On the other hand, many forms exist for viscometers that use the capillary tube, however, most viscometers utilize the differential head caused from gravity as it is disclosed in U.S. Pat. Nos. 6,322,624, 6,402,703, 6,428,488, 6,571,608, 6,624,435, 6,732,573, and 5,257,529, etc. Due to the reason that the viscometer uses the differential head, measurements are limited to liquids, furthermore, issues in the amount of liquids needs exist even if capillary tubes, where the volume of liquids necessary is more than a couple dozen to hundreds of id that is regarded to be a large amount.